See, this is just another reason why Harry is our favourite one. We'd love to have such an attractive grammar nazi in our lives. We'd even get things wrong on purpose, 'n stuff. 
The Directioner took the time to correct the grammar on a fan's sign during a recent gig. Said sign read: "HI HARRY YOUR SO NICE". He asked for the sign to be directed towards the stage, where he grabbed a pen off someone to change it to "HI HARRY YOU'RE SO NICE." Well, he had to entertain himself some how while Niall rambled on in the background. 

He then dutifully sent the sign back to its owner, Taelor (yes, Taelor) Ford, resulting in the below tweet... A pop star's work is never done.